1. Field of the Invention
This invention relates generally to the freezing of liquid material, and pertains more specifically to a method which separates the material into drops and inhibits the agglomeration of the drops during the time they are being cooled on the surface of a cryogenic liquid.
2. Description of the Prior Art
It is often useful to preserve material by cooling the material to temperatures below -50.degree. C for long-term storage. Biological material, such as blood and semen, have presented special problems because of their propensity to agglomerate or coalesce. While most types of biological material can be cooled by a number of cooling methods, the placing of drops of the biological material directly on the surface of a cryogenic liquid, such as liquid nitrogen, has frequently been employed. In this process, the warm drops of material vaporize the cryogenic liquid as they cool, the drops remaining afloat until the cooling is complete, and then the resulting frozen pellets sink below the surface of the cryogenic liquid. The frozen pellets are then stored until needed at which time they can be warmed so as to be in condition for their intended use. This procedure for preserving blood was reported in the Proceedings of the Society of Experimental Biology and Medicine, Volume 90, pages 587-589 by H. T. Meryman and E. Kafig who have found that " . . . by spraying blood in very small droplets (0.45 to 0.9mm in diameter) onto the surface of liquid nitrogen, where they floated momentarily before they sank, and then rewarming the frozen pellets by sprinkling them into plasma or physiological saline at 40.degree. C," they could get 85 to 90% recovery of the blood cells.
In 1960, however, A. P. Rinfret and G. F. Doebbler developed an apparatus utilizing the method of freezing blood that had been reported by Meryman and Kafig. This apparatus sprayed droplets of blood (0.25 to 2.0mm in diameter) onto a moving surface of liquid nitrogen, which permitted a larger scale freezing of blood. Also, the apparatus reduced the tendency of the freezing droplets to coalesce or agglomerate while cooling. The tendency to coalesce or agglomerate has been a common disadvantage as far as this type of cooling method is concerned. If no provisions are made to eliminate the tendency, large irregular clumps of frozen material are formed which do not cool and warm consistently. The problems of freezing and thawing blood, as well as the details of this process, are described in U.S. Pat. No. 3,228,838 granted Jan. 11, 1966, to Rinfret et al. for "Preservation of Biological Substances." The process and the problems associated with the freezing of blood are also lucidly dealt with in an article titled "Observations on the Freezing and Thawing of Blood in Droplet Form" authored by A. P. Rinfret and G. F. Doebbler which appears in the journal Biodynamica, Vol. 8, No. 165 (Nov., 1960), pages 181-193.
In 1971, the need arose to freeze even larger drops (4 to 5mm in diameter) of biological material on the surface of liquid nitrogen, more specifically porcine semen. Inasmuch as these larger drops require a longer period of time to freeze than the smaller drops that have been alluded to above, the coalescing or agglomerating problem became more severe. Concomitantly, the desirability of developing a simpler method of assuring that the drops of material are maintained separate from each other as they froze became more pronounced.